Modern radio receivers are typically equipped with a squelch circuit. The general purpose of a carrier squelch circuit is to mute speaker operation within a radio, cellular telephone, or the like, when the carrier strength of a received signal is weak. A typical carrier squelch circuit consists of a front-end processing unit that determines carrier strength and a detection unit that provides speaker mute control. With reference to FIG. 1, the prior art front-end processing unit 100 may include an amplifier stage 104; pre-emphasis filter stage 106; limiter stage 108; high-pass filter stage 110; rectifier stage 112; attenuation stage 114; and low-pass filter stage 116. The detection unit usually includes a comparator stage 118 for comparing the filtered signal with the preset squelch level in the radio. As is known, the squelch level may be controlled by the radio operator.
To perform effectively, the front-end processing unit 100 must overcome the effects of modulation, fading, temperature, and noise, to name a few. As will be appreciated, advanced squelch circuits may therefore include circuitry for the prevention of reception phenomena associated with temporary changes in the RF signal strength.
Since attack time, the time required to detect a signal, and release time, the time required to affirm the absence of a signal, are important design criteria, the circuit of FIG. 1 performs carrier strength detection and estimation in the high frequency region of the received signal (discriminator output) 102. It is generally held that noise levels in the higher frequency regions provide sufficient dynamic range for detection and fast response times during scan operations.
Squelch circuits sharing the above-topology are typically expensive and demand appreciable real estate resources within any device in which they are used. Despite their high cost and size, these circuits are indispensable mainly due to the significant performance benefits they provide. Despite the benefits, subtle problems associated with the prior art approach are that high frequency components are extremely susceptible to the effects of modulation, especially when the modulation index is large and when the modulating signal is strong. In addition, the best high frequency region for noise squelch has proven to be too high to be suitable for use in narrow band FM radios as well as digital radios with low sampling rates.
It would be extremely advantageous therefore to provide a carrier squelch methodology and apparatus that is small in size in comparison to the prior art, minimizes tuning, reduces power consumption, and is suitable for use with narrow band FM radio receivers.